Digital-to-analog converter parallel-current voltage regulating circuit

ABSTRACT

A parallel-current voltage regulating circuit supplies a compensating current to a load resistor primarily energized from an unregulated source so that the voltage across the load resistor is maintained equal to a reference voltage. Dual regulating circuits are used when the polarity of the voltage across the load is reversible.

United States ate Raamot I DIGITAL-TO-ANALOG CONVERTER PARALLEL-CURRENTVOLTAGE REGULATING CIRCUIT [75] Inventor: EziiiFaErhBtTFiEhEliHTii/bf,

Som eTset Cty., I I .TI Assignee: Western Electric Company,

Incorporated, New York, N .Y.

Filed: Mar. 9, 1971 App]. No.: 122,496

Related U.S. Application Data abandoned.

Division of Ser. No. 805,543, March 10, 1969,

US. Cl.., 307/44, 307/40, 323/22 T, 323/25, 323/80, 340/347 DA Int. Cl.G05f l/56, H031: 13/04 Field of Search 323/1, 16, 17, 19, 323/227, 7,15, 25; 330/69, 97, 105, 106,

m1 avoaeoz Oct. 16, 1973 [56] References Cited UNITED STATES PATENTS3,343,073 9/1967 Mesenhimer 323/15 3,531,598 9/1970 McNair, Jr 323/15 UX2,881,266 4/1959 Miller 330/97 X 3,374,424 3/1968 Wiechmann 323/22 T3,209,277 9/1965 Burwen 330/69 3,239,733 3/1966 Sikorra 330/69 X3,564,444 2/1971 Walsh 330/69 Primary ExaminerA. D. Pellinen Attorney-W.M. Kain [57] ABSTRACT A parallel-current voltage regulating circuitsupplies a compensating current to a load resistor primarily energizedfrom an unregulated source so that the voltage across the load resistoris maintained equal to a reference voltage. Dual regulating circuits areused when the polarity of the voltage across the load is reversible.

2 Claims, 9 Drawing Figures /30;/, (40 SWITCHING PARALLEL CURRENTCIRCUIT REGULATION C RCUIT 2]? (302/, l SWITCHING PARALLEL CURRENT 2CIRCUIT REGULATION CIRCUIT SWITCHING PARALLEL CURRENT CIRCUIT REGULATIONCIRCUIT 4 303 V SWITCHING PARALLEL CURRENT "9 CIRCUIT REGULATION CIRCUITJ I I I (40 4 (40,),

SWITCHING PARALLEL CURRENT CIRCUIT REGULATION CIRCUIT (300/5 SWITCHINPARALLEL CURRENT H9 CIRCUIT REGULATION CIRCUIT PAIINIEIIIII I 8 ma3.7663102 I SHEET 1 UF 3 FIG. m PR/Ufl AR? DIGITAL INPUT s s 5 E0 000OPEN OPEN OPEN O=GROUND ooI OPEN OPEN CLOSED 2/22 EM oIo OPEN CLOSEDOPEN 4/22 E 0| l OPEN CLOSED CLOSED 6/22 E Ioo CLOSED OPEN OPEN 8/22 EIoI CLOSED OPEN CLOSED l0/22 Em. I I0 CLOSED CLOSED OPEN I2/22 E I I ICLOSED CLOSED CLOSED l4/22 E FIG 2 PRIOR ART DIGITAL INPUT n F I: m

"ANALOG ourPur F /6. la

PRIOR ART mum 16 ms SHEET A3 [1? 3 FIG. 6

I06, I03 /04, R /o)2 x AA "V 02, llf [29 [lab "/0! AVAVA' I I (30 1 I40SWITCHING PARALLEL CURRENT CIRCUIT REGULATION cmcun' w (30 1,,

SWITCHING J PARALLEL CURRENT "9 CIRCUIT Y REGULATION CIRCUIT 2 (402/5104 130,), M0 1 3 swncnme J PARALLEL CURRENT CIRCUIT REGULATION CIRCUITm t (30 swncums PARALLEL CURRENT CIRCUIT REGULATION CIRCUIT I (30,)swncume PARALLE cuRRENT CIRCUIT REGULATlON cmcun' SWITCHING PARALLELCURRENT CIRCUIT REGULATION CIRCUIT DIGITAL-TO-ANALOG CONVERTERPARALLEL-CURRENT VOLTAGE REGULATING CIRCUIT This is a division of U.S.Pat. application Ser. No. 805,543, filed Mar. 10, 1969, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a parallel-current voltage regulating circuit and moreparticularly to a parallel current regulating circuit for use with adigital-toanalog converter having a high degree of resolution.

2. Description of the Prior Art A digital-to-analog converter is adevice for converting electrical signals in digital form into theiranalog equivalent. Such convertors are widely used as an interfacebetween a digital computer and a utilizationdevice which cannot bedirectly operated by digital signals. For example, a digital-to-analogconverter is used to convert digital signals generated by a computerinto analog signals to control a machine tool in a computercontrolledmachine-tool operation.

Digital computers are, of course, inherently accurate machines, theiraccuracy being limited solely by the size of the digital words that canbe accommodated in the registers and accumulators of the computer.Double and triple length words may be used for extended precision, insome instances. The commercial digitalto-analog converters which arecurrently available, however, are not capable of converting digitalinput signals into their analog equivalent with the same degree ofresolution inherently possessed by the digital input signals. Prior artdigital-to-analog converters typically comprise a network ofbinary-weighted resistors connected to a common summing resistor. Aseries of switches associated with the binary-weighted resistorsselectively connect a regulated voltage to each resistor in the network,in accordance with the digits of the binary word to be converted. Thecorrespondingly weighted currents-which flow in the binary-weightedresistors are summed in a summing resistor or in a load to produce thedesired analog signal.

Digital-to-analog converters of this type suffer from severaldisadvantages. First, the energizing voltage for the network ofbinary-weighted resistors must be highly regulated. This is difficult todo because the voltage sourcev must supply a considerable amount ofcurrent and must have a relatively high potential, if any reasonabledegree of resolution is required. While voltage sources having a highdegree of regulation accuracy are commercially available, these sourcesare accurate only when supplying a constant or slowly varying load. Whenconnected to a rapidly and constantly varying load, as is found forexample in a digital-to-analog converter, the regulating accuracy dropsto a much lower figure, typically 0.01 percent. This dropin accuracy iscaused by limitations in the regulating circuitry itself as well asuncertainties as to the internal impedance of the voltage source.Furthermore, such a voltage source is relatively expensive and theabove-mentioned limitation on the degree of voltage regulation which canbe obtained of necessity limits the resolution of prior artdigital-to-analog converter to approximately 13 bits. Additionally, inthe prior art, the regulation of the energizing voltage source alwaysoccurs before the source is switched to energize the binary-weightedresistors.

Thus, if electromechanical switching devices are used, contactresistance also becomes limiting. On the other hand, if semiconductorswitching is used, the wide manufacturing variations betweensemiconductors of the same type and the consequent unpredictable voltagedrops through the transistors when conducting, are also limiting. Theuse of field-effect transistors promises to improve this situationslightly, but by not more than one bit, at the very best. Thus,practically speaking, prior art digital-to-analog converters areinherently limited to a resolution of less than 14 bits.

The novel circuitry of the digital-to-analog converter disclosed herein,however, can attain an accuracy and degree of resolution which are bothat least one order of magnitude better than that obtained in the priorart. An experimental 18-bit digital-to-analog converter has beenconstructed and operated. The resolution of this experimental converteris limited only by the state of the art, which at the present time canachieve resistor values and reference voltage sources accurate to onepart per million. As the art progresses it will be possible to increasethe resolution of digital-to analog converters constructed according tothe principles of this invention as well as those constructed accordingto the teachings of the prior art, but the novel circuitry disclosedherein is such that the instant digital-to-analog converter will alwayshave an accuracy at least one order of magnitude greater than the priorart.

A preferred embodiment of the invention comprises a ladder network ofbinary-weighted resistors connected to a summing load. An unregulatedvoltage source is connected through a corresponding series of switchingcircuits to each resistor in the ladder network. These switchingcircuits are selectively energized, in accordance with the digits of thebinary number to be converted, and connect the unregulated voltagesource to each resistor in the binary-weighted ladder network to therebygenerate the analog signal.

An important feature of the invention is the highly accurate regulationof the unregulated energizing voltage after this unregulated voltage hasbeen switched to energize each binary-weighted resistor. This regulationis accomplished by a novel parallel-current regulating circuitcomprising a highly accurate source of reference potential, and anoperational amplifier having a feedback loop connected to supply acompensatory regulating current to the resistor, in parallel with thecurrent supplied by the unregulated source, thereby maintaining thepotential developed across the binaryweighted resistor equal to thepotential of the reference voltage.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1a is a schematic diagram of atypical prior art digital-to-analog converter;

FIG. lb is a chart showing the operation of the converter shown in FIG.la and is helpful in understanding the operation of both the prior artconverter shown in FIG. 1a and the instant invention;

FIG. 2 is a schematic drawing of a portion of another prior artdigital-to-analog converter which illustrates the use of transistorizedswitching circuitry;

FIG. 3a is a schematic drawing of a parallel-current regulating circuitsuitable for use with the present invention;

FIG. 3b is a schematic drawing illustrating an altemative arrangementfor the regulating circuit shown in FIG. 3a;

FIG. 4a is a simplified schematic drawing of the regulating circuitshown in FIG. 3a which is useful in the mathematical analysis thereof;

FIG. 4b is another simplified schematic drawing of the regulatingcircuit shown in FIG. 3a which is also useful in the mathematicalanalysis thereof;

FIG. 5 is a schematic drawing of an alternative embodiment of theparallel-current regulating circuit according to FIG. 3a which provides'an even greater degree of regulation; and

FIG. 6 is a schematic drawing of an illustrative embodiment of adigital-to-analog converter according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1a is a schematic drawingof a 3-bit digital-to-;

analog converter which is typical of the prior art. In this converter aladder network of three binary-weighted S, are shownin their normal, oropen, position and in that position the switches connect ground to eachof the binary-weighted resistors. When operated, how ever, each of theswitches S, through 8;, applies the potential +E to the associatedbinary-weighted resistor causing current flow therethrough. The outputvoltage, E developed across the summing resistor R, may be calculated bytaking the ratio of resistor R, and all binary-weighted resistors inparallel with it to the sum of that quantity just calculated and theparallel combination of all binary-weighted resistors connected throughoperated switches to the reference source. Because the binary-weightedresistors have resistance values which increase according to the powersof 2, the output voltage IE, will increase linearly when switches 8,through S, are selectively closed in; binary order.

FIG. lb'illustrates the operation of the converter in FIG. 1a and showsthe condition of switches S through S, for various. combinations ofinput signals as well as the outputvoltage E developed across thesumming resistor R, for each combnation. This table assumes that R, R.If desired, the circuitry can be arranged so that switches S, through S,switch the binary-weighted resistors between-E and +E,,, rather thanground the +E however, the linear relationship between the analog outputE, and the binary inputwill not be affected. In this latter case thegraph of E, versus the binary input would be shifted downward and wouldpass through the E 0 pointbetween the binary'input numbers 011 and-l00rather than at the normal 000 point.

This type or arrangement is used, for example, if the analog signalis tobe used to control the deflection of an electron beam and it is desiredthat the rest position of the beam be at the center of the cathode rayscreen.

By'increasing the number of binary-weighted resistors in FIG. la, it ispossible to increase the resolution of such a converter. However, asdiscussed previously,

if more than 12 or 13 binary-weighted resistors are connected to theconverter, the change in output voltage E across summing resistor R, asthe last switch, corresponding to the least significant digit of thebinary number to be converted, is operated, is so small that thereference voltage source E must be exceedingly well regulated. This isnecessary so that no voltage perturbations will occur in E which evenapproach in magnitude the voltage change across R, due to a change inthe least significant digit of the binary number.

The cost and difficulty of regulating a power supply for use with adigital-to-analog converter having more than 13 bits has already beendiscussed. The contact resistance of electromechanical switches 8,through S, must also be less than the change in resistance which occursas the last binary-weighted resistor is switched in and out of thecircuit. This, too, limits the resolution 'of the digital-to-analogconverter shown in FIG. la.

As previously discussed, this problem can be alleviated somewhat by theuse of semiconductor switches. FIG. 2 is a schematic diagram of aportion of a commercially available 13-bit digital-to-analog converterwhich uses semi-conductor switching elements. Referring to FIG. 2, whentransistor T, is turned On by the application of a binary one signal tothe base of transistor T the voltage from source E,.,; is applied to thebinary-weighted resistor 2R, as shown. Similarly, the application of abinary zero signal to the base of transistor T, turns transistor T Off"and transistor T, On thus connecting a ground tobinary-weighted'resistor 2R. The alternate operation of transistors T,and T is analogous to the operation of one of the electromechanicalswitches in FIG. 1a. Obviously, the use of transistors in lieu ofelectromechanical switches eliminates the contact resistance problem butintroduces yet another problem in its place. As is well known, when asemiconductor switching device is turned heavily On, it simulates ashort circuit in that the resistance of the emitter-collector path isvery low. There is, nevertheless, a finite voltage drop across aconducting transistor and this voltage drop must be taken into accountin a high resolution digital-to-analog converter. At this time, it isstill not possible to manufacture transistors accurately enough so thatthe collector-emitter voltage drop in the On condition is uniform'fromsample to sample. Thus, difficulty is experienced in calibrating thevarious stages of a digital-to-analog converter which usestransistorized switching and if a transistor failsand must be replaced,the entire converter must be recalibrated, as each stage interacts withevery other stage. Thus, while the use of transistorized switchingcircuits, particularly field-effect transistors, may increase theresolution of prior art converters by one or possibly two bits, thelimitations imposed by the inability to accurately regulate thereference voltage source after switching occurs are still limiting.

The instant invention successfully eliminates the aforementionedproblems and provides I a. digital-toanalog converter whichissignificantly superior to anything heretofore attainable. Thissuperiority is attained by the novel concept of regulating anunregulated voltage sourceafter it has been switched to generate currentflow in a binary-weighted resistor and by using a agesource 12 which mayadvantageously comprise a standard cell having an open-circuit voltageE,- and an internal impedance R,. The standard cell, whose voltage mustbe accurately known, for example to at least one part per million, ismaintained at a constant temperature by the use of an oven. The circuit11 connects the reference voltage source 12 to the input 13 of anoperational amplifier 14 via a resistor R The other input of operationalamplifier 14 is grounded. In the experimental digital-to-analogconverter actually constructed and operated, a commercially availableAnalog Devices Model 21 l operational amplifier was used. This amplifierhas a gain of A an input resistance of 0.5 megohms, an output resistanceof essentially zero ohms, and a noise factor of 10 microvolts r.m.s.Obviously one skilled in the art could substitute other types ofoperational amplifiers for the one used in the experimental embodimentprovided that the characteristics are similar. The output 15 ofoperational amplifier 14 is connected by a feedback loop 16 to the input13 thereof. An output resistor R and a resistor R are connected in thefeedback loop. R the load resistor to be energized, is connected betweenground and the juncture of resistors R and R in feedback loop 16. Asecond circuit 18 connects an unregulated voltage from an externalvoltage source 19 to the juncture 17 of resistorsR, and R via a resistorR The unregulated voltage source 19 advantageously comprises anysuitable source having a voltage E, and an internal impedance 1) Inoperation, the unregulated voltage source 19 supplies a current throughR, and R to ground, and the voltage E which isdeveloped across loadresistor R at juncture 17 is regulated so that it equals the opencircuit voltage E,- of reference voltage source 12 to a high degree ofaccuracy, regardless of wide variations in the magnitude of the voltagefrom unregulated voltage source 19 If, as in the illustrativeembodiment, the potential of reference source 12 is known within onepart per million then resistors R R and R must also be accurate towithinone part per million or better.

Assume that unregulated voltage source 19 is supplying a current i toload resistor R and that the voltage developed across juncture 17 is E,.The potential of unregulated voltage source 19 is seen by the input ofoperational amplifier 14 through resistors R and R,, R being in feedbackloop 16. Input 13 of operational amplifier 14 also sees" the potentialfrom reference voltage source 12 through resistor R In order that theoperational amplifier will regulate over the desired range, themagnitude of resistors R and R are selected such that R isapproximatelyequivalent to the series combination of R; and the internal resistanceR,- of reference voltage source 12. Typically, R, is negligibly smalland, thus in practice, R will equal R If the potential of unregulatedvoltage source 19 should vary for any reason, the magnitude of voltageE, will also tend to vary. This variation is sensed at the input ofoperational amplifier 14 and an output current is generated in feedbackloop 16. A fraction, i,,, of this current will flow through loadresistor R Since i is in parallel with i, the current supplied fro'mfeedback loop 16 will tend to alter the magnitude of E,, in anoffsetting manner, and maintain E, equal to the voltage of referencevoltage source 12. For this to occur, it is necessary that the magnitudeof resistor R, be selected so that its resistance, together with theinternal impedance R, of unregulated voltage source 19, approximatelyequals the resistance of load resistor R,,. Load resistor R will, ofcourse, vary in resistance for each stage of the digital-to-analogconverter.

A mathematical analysis of this circuit is set forth below and thisanalysis proves that the parallel-current regulating circuit shown inFIG. 3 doe indeed maintain the voltage E, developed across load resistorR equal to the voltage of reference voltage source 12.

FIG. v4a shows the circuit of FIG. 3a redrawn in simplified form. Theinternal resistance R, of reference voltage source 12 and the internalresistance R, of unregulated voltage source 19 have been ignored, astypically they are very small compared to R and R respectively. If theyare not small compared to R and R the following mathematical analysis isstill valid provided that R and R, are substituted in the followingproof where R =R2+R and R4 =R +R FIG. 40 itself may be furthersimplified, as shown in FIG.'4b, by defining two new quantities R and Ewhere and ET=RL/R4+RL E Assume that currents i i and i flow as shown inFIG. 4b. No current is shown flowing into the operational amplifier asit normally requires no input current.

Now the currents i i and i may be found from the following equations.

i2 Eo E /Ro p and i3=E E /RT.

But

E -A E,

and

and from Equations and 10 E, (A 11,112, 11, E, A R /R, R,) E,

From Equations 12 and 13, we obtain and v R, i 300 ohms R. a... a 500ohms Thus E, (E,[l0 (200)(l0 )]/[l400 +10 200 (l0')]) (E (10O)/l40 l 20+10 Therefore E r T/ Ignoring this latter term, which is exceedinglysmall, we get E, E,. and the effect of E (and thus E,,) upon E, may beignored. v

Another important feature of the parallel-current regulating circuitshown in FIG. 3a is its excellent response to transient switchingpulses. An experimental regulator, constructed in accordance with thisinvention, was found to have a settling time oflessthan 1 microsecond.This figure is suitable for virtually all applications which require adigital-to-analog converter of this degree of resolution.

FIG. 3b shows a modified version of the circuit disclosed in FIG. 3a. InFIG. 3b a diode 21 is connected between output lead 15 and input 13 andpoled out so. that if the output voltage E of operational amplifier 14goes positive, diode 21 conducts shorting out the operational amplifier14. Obviously, diode 21 could be poled in the opposite direction if itis desired to shortout operational amplifier 14 when the output voltagegoes negative.

It is possible to improve still further the parallelcurrent regulatingcircuit shown in FIG. 3a. As shown in FIG. 5, this is accomplished bysubstituting for the external reference voltage source 12in FIG. 3a, anunregulated voltage source which itself has been regulated by aparallel-currentregulating circuit of the type disclosed in FIG. 3a.This process may be repeated over and over again althoughpracticalconsiderations make the use of more than two parallelcurrent regulatingcircuits in tandem of doubtful value. Referring now to FIG. 5,-a circuit11 connects a reference voltage source 12 toa resistor'R, thence to-theinput 13 of an operational amplifier 14, A feedback loop l6 whichincludes an output resistor R," and a resistor R,

connects the output 15 of operational amplifier 14 to the input 16. Aload resistor R is connected to the juncture 17 of resistors R and R,and is supplied with current from an external unregulated source 19through a resistor R The operation of the portion of the circuitdisclosed in 'FIG. 5 described so far is identical with that describedfor FIG. 3a and will not be repeated here. Suffice it to say that thevoltage E, developed across load resistor R is held to the voltage ofreference source 12. Importantly, no significant current is drawn fromthe standard cell in reference source 12 The regulated voltage E,developed across resistor R,, is next connected via circuit 11 andresistor R to the input of a second operational amplifier 14 and theremainder of its circuit and the operation thereof identical to thatdescribed with reference to FIG. 3a. The circuit of FIG. 5 accomplishesthe substitution of E}, the voltage developed across resistor R,,, forthe reference voltage source 12 normally used. Unlike a standard cell,however, the circuit arrangement of FIG. 5 can supply significantcurrent from the new reference source E, which increases the range andaccuracy of overall regulation.

FIG. 6 shows a preferred embodiment of a digital-toanalog converteraccording to the invention. With the exception of minor changes in theresistance of some components caused by a corresponding change in theresistance of the associated binary-weighted resistor, each stage of theconverter is identical, thus only the first is illustrated and describedin detail.

As discussed in connection with FIG. 2, in the preferred embodiment, thepotential applied to the various biriaryeighted resistors in the laddernetwork is switched between +E and E,,. However, it will be appreciatedthat the circuitry disclosed is equally suitable for switching between:13, and ground, should this be desired. If this were done, it would, ofcourse, be necessary to regulate the ground potential, as well as thesupply source, if a high degree of resolution is required.

In FIG. 6 a ladder network 101 comprising a plurality of binary-weightedresistors R, 2R, 4R 2" R is connected' at one end to a summing circuit102 hence to an external load 103 which, in the preferred embodimentshown, is the deflection coil of an electron-beam deflection device. Itwill be appreciated, however, that load 103 may be any external loadcapable of working with the digital-to-analog converter disclosed.

Input 104 is connected by leads 106a and 106b to essentially identicalupper and lower halves of the first stage of the converter. Again, forsimplicity, only the upper half of the first stage will be discussed indetail, although both are illustrated. The circuitry and operation ofthe lower half is entirely analogous to the operation of the upper half,provided, of course, that appropriate corrections are made for thechange in polarity of the supply source.

Returning now to the input 104,, lead 106a connects the input via aresistor 107a and a capacitor 108a to the base of transistor Qla.Resistor 109a connects biasing potential from source -E,, to the base oftransistor Qla. The emitter of transistor Qla is connected to a secondbiasing potential 'E via a lead 111a. The collector of transistor Qla isconnected via a resistor 1 12a and at capacitor 1 13a to the base oftransistor Q2a. A biasing resistor. 114a connects the base of transistorQ2a to its emitter and thence via a lead 116a to an unregulated voltagesource +E,,. The collector of transistor 02a is connected by a resistor117a to a feedback loop 118a. a junction 119 and the firstbinary-weighted resistor R.

The concept of regulating the energizing voltage sources +E and -E,,after they have been selectively switched to energize thebinary-weighted resistors in ladder network 101 has been previouslydiscussed. To accomplish this regulation, after switching, anoperational amplifier 121a having an input 122a and an output 123a isconnected via an output resistor 124a and the serial connection of adiode 126a and a Zenerdiode 127a to feedback loop 118a. A resistor 128aconnects the junction 129a of feedback loop 118a and a resistor 117a toone end of a trimmer resistor 130a and via the slider arm thereof toinput 122a. The other end of trimmer resistor 130a is connected via aresistor a to an external reference voltage E,,;. The output 123a ofoperational amplifier 121a is also connected via a pair of diodes 131aand 132a to the input 122a thereof. A resistor 133a connects themidpoint of diodes 131a and 132a to ground.

As previously discussed, the lower half of the first converter stage isessentially identical to the upper half just described. However, it willbe noted that the polarity of transistors Qla and Qlb and Q2a and 02bare reversed. Similarly, lead lllb connects the emitter of transistorQlb to ground rather than to the biasing voltage E and resistor 10%connects the base of transistor Qlb to the biasing potential +E ratherthan E,. Similarly, lead 116b connects the emitter of transistor Q2b toE,, rather than +E and the polarity of diodes 126b, 131b, 132b and Zenerdiode 127b are also re-' versed.

In operation, assume that a very low potential, illustratively zerovolts, is applied to input 104, and is indicative of a binary one inputto the first stage of the converter. Because the emitter of transistorQlb is grounded, when zero volts is applied to the base thereof therewill be, of course, zero volts on the base-emitter junction oftransistor Qlb. Thus, the large positive potential +E which is appliedto the base of transistor Qlb via biasing resistor 10% maintainstransistor Qlb positively turned Off. This in turn keeps transistor Q2bOff which inhibits the application of the -E,, voltage present on lead116b to the junction point 1 19 and binary-weighted resistor R.

Resistors 107a and l07b prevent transistors Qla and Qlb from loadingdown the input signal on input 104 and capacitors 108a and 108b areprovided to speed-up the transient response of transistors Qla and Qlb.The zero potential which is applied, via lead 106a, to the base oftransistor Qla turns transistor Qla heavily On, by virtue of thenegative potential E connected to the emitter thereof by lead 11 1a. Theresistor 109a which connects the base of transistor Qla to biasingsource E,, plays no part in the operation of the circuit at this time.This connection is provided to ensure that transistor Qla is positivelyturned Off when the input on lead 104 corresponds to a binary zero andtransistors Qlb and 02b are conducting.

When transistor Qla is turned On the potential at the collector thereofbecomes approximately equal to the negative potential -E,, which ispresent on its emitter. The base of transistor Q2a on the other hand, ismaintained positive because of the positive potential +E,, present onits emitter via lead 116a. Current therefore flows down throughcurrent-limiting resistor 112a into the now conducting transistor Qlaand this downward current flow draws current from the base of transistor02a and turns transistor Q2a heavily On permitting current to flow fromsource +E,, through resistor 117a to jun ction 119 and the firstbinary-weighted resistor R, thence via summing circuit 102 to the Thevalue of resistor 1T2 a is selected to limit the current flow throughtransistor Ola to a reasonable figure and capacitor 113a is provided tospeed-up the transient response of transistors Qla and 02a.

As previously discussed with reference to FIG. 3a, the potential atjunction 129a is regulated by means of operational amplifier 121a andfeedback loop 118a so that the voltage at junction 129a is maintainedequal to the potential of reference voltage -E,,;. Trimmer resistor130a, which istypically a 20-turn wire wound potentiometer ofapproximately 1 ohm resistance, is provided for alignment purposes sothat the potential at input 122a to operational amplifier 121a ismaintained at volts.

Also as previously discussed in connection with FIG. 3a, if forsomereason theIpotential at junction 129a should vary from the referencepotential E,,,, a compensating, parallel-current is supplied to junction119 and binary resistor R from feedback loop 1 18a to maintain thevoltage at junction 129a and hence junction 119 constant. The Zenerdiode 127a, which is connected in the feedback loop 118a, is provided toin-' crease the range of regulation and this diode drops approximately75 percent of the voltage difference between the output of operationalamplifier 121a, which is typically 5 volts, and the potentialat junction119,, which is typically volts, the remaining 5 volts being droppedacross'resistor 124a. Diode 126a is provided to block leakage currentfrom operational amplifier 121a when current is being supplied toresistor R by the lower half of the circuit, which, of course, onlyoccurs when a binary zeroi signal is present at input 104, Diodes13laand 132a and resistor 133a are connected across operational amplifier121a to short it out in this latter condition when operational amplifier12111 is functioning.

The operation of the circuit when a binary zero signal is applied toinput 104; is entirely analogous. The

potential applied to input 104, in that case is typically 3 volts, thusthe biasing potential E,, applied to the E,, potential on lead 1 16b tojunction 1 19 and binaryweighted resistor R thence via summing circuit102 to the load 103 as before. Operational amplifier 121k and ;feedbackloop 118b act to maintain the potential at junction 12% equal to thepotential +E,-,,applied to the input of operational amplifier 121b, in amanner entirely analogous to-the operation of the upper half of vthecircuit. I I

' As previously discussed, under this condition, opera tional amplifier121a will be shorted by diodes 132a 12 The converter stage which hasbeen illustrated and discussed in detail is the most significant stagein the .converter. The stages which correspond to the lesser significantdigits of the number to be converted are essentially identical eMa3forminor changes in resistance values, etc. Thus, the application of inputsignals to input circuits 104 through l04,N 1 and the operation ofswitching circuits 30 through 30, and 30 through 30, as well asparallel-current regulating circuits 40 through 40 and 40 5 through 40,need not be discussed in detail.

It is essential, in the first stage of the converter, that resistors a,125b, 128a, l28b as well as binaryweighted resistor R be as accurate asthe potential of the reference voltage +E and E,,,. In the illustrativeembodiment of the invention actually constructed, these potentials wereknown to within one part per million. In succeeding less-significantstages of the converter, these tolerances may be reduced, for example,to two parts per million, four parts per million, eight parts'permillion, etc. As previously discussed, the reference voltages +13 and-E,,, are advantageously obtained from a standard cell. The standardcell is advantageously maintained in a controlled environment in atemperature controlled oven. It may also be advantageous, under somecircumstances, to maintain resistors 125a, 125b, 128a and 128k and thebinary-weighted resistors in a temperature controlled oven, althoughthis features of the invention according to the preferred embodiment, itwill be understood that various omissions and substitutions in thecircuitry illustrated and in its operation may be made by those skilledin the art without department from the spirit of the invention.

what isciainied is:

1. A circuit for regulating the voltage across a load, the load beingconnected by a resistive circuit to a source of reversible-polarityenergizing voltage, which comprises: I

circuitry supplying a positive reference voltage;

a first operational amplifier having a first inputconnected to groundpotential, a second input and an output; v

a first resistor connecting said positive reference circuitry to saidsecond input of said first amplifier;

a second resistor substantially equal in magnitude to said firstresistor, connecting said load to said sec- 0nd input of said firstamplifier;

a first feedback circuit connecting said output of said first amplifierto said load to supply a first compensating current to said load whensaid load voltage is negative to maintain said load voltage substan-'tially equal in magnitude to said positive reference voltage, regardlessof variations in the'magnitudeof said energizing voltage; a first diodeconnected between said output and said second input of said firstamplifier, and poled to conduct when said load voltage is positive todisable said first amplifier;

circuitry supplying a negative reference voltage;

a second operational amplifier having a first input connected to groundpotential, a second input and an output;

a third resistor connecting said negative reference circuitry to saidsecond input of said second amplifier;

a fourth resistor, substantially equal in magnitude to said thirdresistor, connecting said load to said second input of said secondamplifier;

a second feedback circuit connecting said output of said secondamplifier to said load to'supply a second compensating current to saidload when said load voltage is positive, to maintain said load voltagesubstantially equal in magnitude to said negative reference voltage,regardless of variations in the magnitude of said energizing voltage;and

a second diode, connected between said output and said second input ofsaid second amplifier, and poled to conduct when said load voltage ispositive to disable said second amplifier.

2. A circuit for regulating the voltage across a load,

the load being connected by a resistive circuit to a source ofreversible-polarity energizing voltage, which comprises:

circuitry supplying a positive reference voltage;

means for comparing the voltage across said load with said positivereference voltage to obtain a first signal related to the differencebetween said load voltage and said positive reference voltage;

means for supplying a first compensating current to said load accordingto said first signal so that the effect of said first compensatingcurrent on said load tends to decrease said first signal;

means for disabling said first current supplying means when said loadvoltage is a second polarity, opposite to said first polarity;

circuitry supplying a negative reference voltage;

means for comparing the voltage across said load with said negativereference voltage to obtain a second signal related to the differencebetween said load voltage and said negative reference voltage;

means for supplying a second compensating current to said load accordingto said second signal so that the effect of said second compensatingcurrent on said load tends to decrease said second signal; and

means for disabling said second current supplying means when said loadvoltage is said first polarity.

ES PATENTOFFICE RECTION l gzcrkNo. 3,7 6,4 2 Dmd October 16, 1973lhv'emofls) 7 JAAN' WOT I D It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown beiow:

a Title page and Column "1, the title "DIGITAL-TO-ANALOG w CONVERTER"PARALLELCURRENT VOLTAGE REGULATING CIRCUIT" should p read-PARALLEL-CUFRRENT VOLTAGE REGULATING CIRCUIT '1'; p In the Abstract,line 1, after "circuit" insert "for use with a digitalto-analogconverter--. I

In the specification, Column 1, line '12, after "current" insertvoltageq line 67-, "before" should read --before--. Column ,2, line '43,"after" should read "after- Column 3, 7 line" 593' "type .Or" shouldread --type of--. Column L, line 61, "-after"-should read -after--.Column 5, line 39, "known within" should read known to within--. Column6 line 7 "FIG. .3 doe' should read --FIG. 3 does--. Column line l-T,poled out so" should read --poled so--. Column ll, line 52, after' -"-3"insert --volt--. Column 12, line 5, "identical eMa3for minor". shouldread --identical except for minor--;

line "depa rtment" should read "departing- S ig'nedl and sealed this12th day of March 1974.

TgssALj" 1 EDWARD.,MQFLET'CHERJR. c x c. MARSHALL DANN Attesting OfficerL Commissioner of Patents *UNrr-ATESPATENTOFF E 0v CORRECTION $7 6M M'Oto'ber 1&1973' \J 'lnvemods) IAAN B -M It is certified that errorappears in (heabove-idemified patent and that said Letters Patent arehereby corrected as shown beiow:

F 1 Title page and Column "1, the title "DIGITALTO-ANALOGCONVERTER'PABALLEL-CURRENT VOLTAGE REGULATING-CIRCUIT should i readPARALLELCU RRENT VOLTAGE REGULATING CIRCUIT--.

P; In the Abstract, line 1, after "circuit" insert --for" use with adigital- -to-analog converter-.

In the specification, Column 1, line '12, after "current" insert--voltage-; line 67, "before" should read --before-. Column 2, line #3,"after" should read --after--'-". Column 3, line-'59, "type .or""sho.uldread "type of--. Column 4, line 61, "after" should read --after--.Column 5, line 39, "known within" should read --known to within--.Column line 7 "FIG. 3 doe" should read -FIG. '3 does--. Column ,'linei7, "poled out so" should read -poled so--. Column 11, line 52, afterf"-3" insert --volt--. Column l2,- line 5,- "identical eMaBfor minor"should read --identica.l except for minor-.;

line ML, "department" should read '--departing- ""S ig'nedtand sealedthis 12th day of i'l (sEALj Attjestz v p v EDWARD ,-M.*FLETCHER,JR.Y' c.MARSHALL DANN Attesting Officer I Commissioner of Patents

1. A circuit for regulating the voltage across a load, the load beingconnected by a resistive circuit to a source of reversiblepolarityenergizing voltage, which comprises: circuitry supplying a positivereference voltage; a first operational amplifier having a first inputconnected to ground potential, a second input and an output; a firstresistor connecting said positive reference circuitry to said secondinput of said first amplifier; a second resistor substantially equal inmagnitude to said first resistor, connecting said load to said secondinput of said first amplifier; a first feedback circuit connecting saidoutput of said first amplifier to said load to supply a firstcompensating current to said load when said load voltage is negative tomaintain said load voltage substantially equal in magnitude to saidpositive reference voltage, regardless of variations in the magnitude ofsaid energizing voltage; a first diode connected between said output andsaid second input of said first amplifier, and poled to conduct whensaid load voltage is positive to disable said first amplifier; circuitrysupplying a negative reference voltage; a second operational amplifierhaving a first input connected to ground potential, a second input andan output; a third resistor connecting said negative reference circuitryto said second input of said second amplifier; a fourth resistor,substantially equal in magnitude to said third resistor, connecting saidload to said second input of said second amplifier; a second feedbackcircuit connecting said output of said second amplifier to said load tosupply a second compensating current to said load when said load voltageis positive, to maintain said load voltage substantially equal inmagnitude to said negative reference voltage, regardless of variationsin the magnitude of said energizing voltage; and a second diode,connected between said output and said second input of said secondamplifier, and poled to conduct when said load voltage is positive todisable said second amplifier.
 2. A circuit for regulating the voltageacross a load, the load being connected by a resistive circuit to asource of reversible-polarity energizing voltage, which comprises:circuitry supplying a positive reference voltage; means for comparingthe voltage across said load with said positive reference voltage toobtain a first signal related to the difference between said loadvoltage and said positive reference voltage; means for supplying a firstcompensating current to said load according to said first signal so thatthe effect of said first compensating current on said load tends todecrease said first signal; means for disabling said first currentsupplying means when said load voltage is a second polarity, opposite tosaid first polarity; circuitry supplying a negative reference voltage;means for comparing the voltage across said load with said negativereference voltage to obtain a second signal related to the differencebetween said load voltage and said negative reference voltage; means forsupplying a second compensating current to said load according to saidsecond signal so that the effect of said second compensating current onsaid load tends to decrease said second signal; and means for disablingsaid second current supplying means when said load voltage is said firstpolarity.